In July 2004 a swarm of deep earthquakes began, and continued until the end of the year.

With a seismograph they determine that the rumbles are due to some kind of surface activity, not deep earthquakes.

Recent geological studies (particularly the examination of very small and deep earthquakes) suggest that the two faults may be connected.

Thus, deep earthquakes are usually not of interest for alerts.

It is the first physical evidence that deep earthquakes are different from shallower ones.

Scientists have long known that deep earthquakes could not occur in the same way because of high pressures deep underground.

"It's the first real difference in the physical characteristics of deep earthquakes," said Dr. Williams.

The scientists say that deeper earthquakes may be caused by a mechanism called phase transformation.

The eruption was preceded by a 60 km deep earthquake on 27 January.

The quake also destroyed scientists' opinions on deep earthquakes.